Multichannel system



Sept. 24, 1946. E.A|.AB1N

I MULTIGHANNEL SYSTEI 3 Sheets-S0, 1

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,Sept 24, 19.46. E. LABIN HULTICHANNEL SYSTEM Filed Aug. 25, 1944 3 Sheets-Sheet 2 CLIP/ER n/ E www /Ww mw fm mmf@ W W w /L M fp. fe H y mpm Pw Ps A TT 0PM' Y 3 Sheets-Sheet 5 E.. LABIN MULTI CHANNEL SYS TEM Filed Aug. 25, 1944 Sept. 24,- 1946.

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Patented Sept. 24, 1946 MULTICHANNEL SYSTEM Emile Labin, New York, N. Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application August 25, 1944, Serial No. 551,202

I Thisinvention relates to multi-channel communicating system and more particularly to systems for transmitting in interleaved manner the time modulated pulses of a plurality of channels.

One of the objects of the invention is to provide a multi-channel transmitter and method for either multi-channel transmission to a second 15 Claims. (Cl. 179-15) 2 plaining the operation of the embodiments shown in Fig. 4. j

The multi-channel transmitter shown in Fig.V 1 may comprise-the Vtransmitting equipment for a two-way multi-channel communicating system or` it may be usedas a multi-channel broadcasting system for selective reception of the channels by terminalor for multi-channel broadcasting from Y a given station for selective reception by individual receivers.

Another object of the invention is to provide a new method and means for time modulating a train of pulses.

In most multi-channel communicating systems employing pulse time modulation, each channel is provided with an individual time modulator. VAccording to one Vof the features of my invention I provide a single time modulator for all of the channels. A pulse train corresponding to the interleaved pulses vof all the channels is applied to kthe modulator and the signal energy of each channel is divided into spaced pulse segments timedv diiferently for the diiferent channels, the pulse segments of the signal energy of all the channels being applied in interleaved manner to the'time modulator as the modulating energy. The pulses of the train are displaced in time in accordance with the amplitude of the corresponding pulse segments. 'I'his method of modulation may of course be employed for a single channel where desired, one of the advantages of such a system for a single channel, or even for multichannel transmission, being that the interval between pulse segments of any one channel may be blocked at the receiver thereby insuring a high signal-to-noise ratio. Another advantage of the system particularly in multi-channel operation is a reduction in the number of time modulators required by such systems heretofore proposed.

For a lfurther understanding of the invention, reference will be had to the following detailed description to be read in connection with the accompanying drawings, in which:

Fig. 1 is a block diagram of a multi-channel transmitter'in accordance with the principles of this invention;

Fig. 2 is a graphical illustration useful in explaining the operation of the transmitter of Fig. 1;

Fig. 3 isa schematic Vwiring diagram of a pulse time modulator that may be yused in the system of Fig. 1; i e

Y Fig. 4 is a block diagram of a further ment of the invention, and Y Fig. 5 is a graphical illustration used inexembodiindividual receivers. Y In the case of multi-channel broadcasting the transmitter, -of course, will be provided with a suitable omni-directional kantenna. For simplicity of illustration the system is shown for three transmitting channels, I, 2 and 3, although many additional channels may be included, if desired. v

The embodiment shownin Fig. 1 includes aV stable type ofpulse producer 4 of known form for producing a train of pulses such as indicated at 5 in graph a of Fig. 2. The repetition rate of the pulses 5 is selected in this embodiment as that pulse rate desired for a single channel. The pulses 5 are multiplied according to the number of channels desired by applying the pulses to a pulse multiplier 6 of known character which may shaped, if desired by applying pulses 5 to pulse shaper 8a through connection 8b. In the present showing, however, the same pulse. width is used for synchronizing purposes and for selecting seg.- ments of signal energy. The output pulses 9 Vof shaper 8 areapplied to a delay network I0 having output connections I I spaced' at diiferent retardation points therealong whereby pulse energy may be supplied at -dierent time intervals corresponding to the timing desired for pulses of the diierent channels. The output connection for channel I is applied to an amplifier I2V which is normally blocked. The signal source I3 for channel I is also applied to the amplifier I2 but the blocking potential for the Yamplifier is so chosen that ythe signal energy remains blocked regardless of its variations in amplitude.L The retarded or keying pulses from output connection Ilaunblock the amplifier for the duration of-each of the pulses.

`This 'passes a segment of the signalv energy, or

rather producesin effect an output pulse lwhich is amplitude modulated in accordance with the 3 segment of signal energy coincident with each keying pulse.

In graph b, for example, the pulses I4, I5 and I6 represent three successive output pulses from amplifier i2 which are representative segments f the signal envelope I'I. These signal pulses thus define points along the signal envelope I1 as indicated.

The output pulses of each of the amplifiers of the different channels are applied over connection I8 to a time modulator I9 to which is also applied the pulses l, graph c. The signal pulses of the dii-ferent channels are interleaved in` accordance with the retardation applied thereto in delay device le. Pulses I4, .I and I6 are shown to be retarded an amount t1 causing them to Coincide with the pulses of channel I. For channel 2 the pulses 9 are retarded an amount t2 for coincidence with pulses of channel 2. It will be clear, therefore, that the amplitude modulated pulses of each channel coincide in time With pulses of the pulse train 1 of graph c.

The time modulator I9 may be of any desired character capable of displacing in time the pulses I in accordance with the amplitude 4of the corresponding pulses applied over connection I8. Two examples of time modulators of this character are hereinafter described in connection With Figs. 3 and 4. v

Graph e of Fig. 2 illustrates the time displacement produced on the pulses of channel I by the pulses I 4, I5 and. I6 of graph d. The loW or minimum amplitude of pulse I 4, which may be regarded as corresponding to the maximum negative signal value, is indicated as having retarded pulse Ia of channel I an amount T. This minimum amount of retardation for pulse I4 may be regarded as always present in the time modulator. The amplitudes of pulses I5 and I6 are shown to time displace the pulses Ib and Ic amounts T-l-At and T-1-2At respectively, since they are of successively greater signal values. It will be clear that the time displacements of the pulses of the several channels vary in amounts according to the instantaneous values of the corresponding signal energies, and that the displacements are relative to the pulse positions of train 'I in the absence of modulation or with respect to the positions of the pulses S of the synchronizing channel (graphs f and g).

The pulse energy 9 from Shaper 3 is applied over connection 29 to a clipper mixer 2l to which is also applied the time modulated pulse output of modulator I8. The synchronizing pulses being the same as pulses 9 in graph b as to Width and timing coincide with pulses of a fourth channel in the train of pulses l. By providing the mixer 2l With a limit clipping level indicated at 22 in graph f the pulses 4 are replaced by synchronizing pulses S, the resulting train of pulses for transmission being as shown by graph g. These pulses are applied to transmitter 23 for transmission at a given carrier frequency over antenna 24 which may be of a directional or omni-directional type.

Referring to Fig. 3 a pulse time modulator is shown that may be employed as the time modulator I9 in Fig, 1. The time modulator comprises a retardation circuit 25 having an inductance coil 26 with a core of iron or other permeable material, and condensers 21, 28.

The minimum retardation characteristic of the circuit 25 may be represented by the time interval T of graph e, Fig. 2 for a maximum negative signal value such as indicated by pulse I4. The

signal pulses of graph d which vary in amplitude in accordance with segments of the signal energy are applied over connection I8 to the anode circuit 29 of an amplifier 30 to Which is applied over connection BI the pulses 'I' from pulse multiplier 5. The source of anode voltage is applied through the resistor 32 and the retardation circuit 25. Should the impedance of coil 26 be sufficient the resistor 32 may be omitted.

The instantaneous signal value of the signal envelope Il at the occurrence of pulse I5 is reater than the minimum value at the occurrence of pulse I4, and therefore produces greater retardation eiect on pulse Ib by a total amount equal to T`-|./ \.t. This greater retardation of the pulses I b over pulses Ia is produced by a change in permeability of the core of coil 26, which alters the inductance of the circuit 25. The still greater signal value represented by pulse I produces a still greater retardation for pulses ic which equal to T-1-2At. From the foregoing it is believed clear that as the signal energy increases still further, the greater the time displacement Will be for the corresponding pulses of channel I. The time displacement according to this method of modulation is of course small but small displacements are of advantage in multi-channel communication since a larger number of pulses is possible for a given time interval. For a further discussion of this principle of time modulation reference may be had to my copending application Serial No. 546,378 filed July 24, 1944.

In Fig. Ll show an embodiment of the invention wherein pulse producer 33 is arranged to produce pulses .at the repetition frequency desired for all of the channels taken together. The pulse output of producer 33 is represented by the pulse train 34 in Vgraph m of Fig. 5. To obtain the pulses for each channel, the pulse train 34 is applied t0 a pulse divider 35 whereby pulses 36, graph 11, are produced. These pulses are then applied through a pulse shaper 31 to a delay net- Work 38 similar to the device I0 in Fig. l. The pulse energy 3S is thus shaped and delayed for channel I as indicated by the pulses 39, 40 and 4I of graph o. These Wider delayed pulses are applied to channel amplifier 42 for selecting a segment of signal energy from source 43 in the manner hereinbefore described. The output pulses 39, 49, 4I of amplifier 42 are thereafter applied to a clipper gate 44, for translation of the amplitude modulation thereof into time modulation.

If a time modulator of the character shown in Fig. 3 were employed in the system of Fig. 4, the pulses 34 would be applied directly thereto as hereinbefore described in connection with Fig. l. Before application of pulse energy 34 to clipper gate 44, it is desirable to reshape the pulses into pulses each having at least one inclined edge corresponding in time to one of the pulses of the several channels. The energy of the pulse train 34 is therefore applied t0 a Wave producer 45 of the type capable of producing a zig-Zag shaped Wave 4B, the undulations of which are shown in graph p, symmetrically shaped. The wave, of course, may be varied considerably, and if desired may be of the saw-tooth form having one edge of each tooth inclined and the other edge thereof substantially vertical.

Various types of clipper gates may be employed at 44, the example shown being of a type having two dry rectiiiers 47 and 48. The two rectiers are connected back to back and a small positive voltage is applied over resistor 49 to the interwcorresponding greater time displacement.

mediate point 50. The resistor 49 is preferably at least several times larger than the low forward resistance of the rectiers, although it should be low in comparison with'the high back resistance thereof. The input wave 46, graph p is applied through coupling *condenserk 5|` to resistor 52 which is preferably several times smaller thanA resistor 49. The output of the circuitV is delivered across resistor 53 which maybe of the same order of magnitude as resistor 52,

For modulating the clipping action the amplitude modulated pulses such as 39, 40, 4|, etc., of channel I are applied through resistorY 52 to rectifier 41. The relationship between clipping levels 41a, 48a of the clipper gate vand wave 46 shown in alignment with the pulses of the fourth channel, graphs m and p, Fig. 5, represents the condition when the amplitude modulated pulse input is ofan amplitude corresponding to the maximum negative value of the signal energy for such channel. The action of the two rectiers Y is that of limiting the passage of input wave 46 between levels 41a and 48a. The application of energy to -the circuit through resistor 52 operates to vary the location'of these two levels relative to the amplitude Yof the wave 46.

This double limit clipping results in Va rectangular or trapezoidal wave form suchv as indicated in graph q. This rectangular wave is applied to a diierentiator 54 which converts each rectangular pulse portion of the wave into alternately positive and negative pulses as indicated by graph r. It will be noted that the rectangular pulse portion 55 corresponding to the clipping of wave 46 in alignment ywith channel 4 produces a narrow pulse which, when generated, producesY :are separated by a time interval corresponding to the width of pulse 51. Likewise pulses 58d and 58h resulting from pulse 58 are displaced a greater time interval corresponding to the greater amplitude of pulse 46, and the still greater pulse width 59 resulting from the clipping produced by pulse 4| produces pulses 59a and 59h having a 'I'he pulses of graph r are applied to a threshold clipper circuit 6B by which pulses 551), 51h, 58h, etc., are clipped at level 6l. Of course, pulses 55a, 51a, 58a, etc., may instead be clipped whereby the time displacement of the output pulses will be in the opposite direction.

The output of the clipper 60 is applied to a mixer 62 to which is also applied synchronizing pulses 64 obtained by shaping the pulse output of pulse divider 35 at shaper 63. The synchronizing pulses 64 are shown in graph s to coincide with pulses 55h produced from the pulses of channel 4, and since the mixer 62 is preferably of the character having a limit clipping level 65, the synchronizing pulses 64 replacethe pulses 55b. This results in a train of pulses indicated by graph t in which the synchronizing pulses 64 are of a width greater than the pulses of the channels I, 2 and 3. This greater width of the synchronizing pulses makes it possible for the receiver to segregate the synchronizing pulses from the channel pulses by width discrimination for Vusein deblocking circuits of` the receiving channels. 1 t

For a more complete discussion of the time modulating principles of the clipper gate type of r5 VmodulatorV and of variations thereof, reference may be had to the copending application of D. D. Grieg and myself, Serial No. 455,898, filed August 24, 1942. f

From the foregoing description it is clear that 10 I have disclosed a multi-channel communicating system requiring a single time modulator for a plurality of channels. Also, I have shown two diierent methods of producing the train of pulsesA to vbe time modulated and a second train of pulses, energy ofwhich is used for each of the channels to obtain segments of the signal energy of the different channels. While I have indicated a base pulse producer from which these two trains are obtained it will be clear that in the'place of a base pulse producer I may use a stableY oscillator for producing a sinusoidal wave havingthe frequency desired for either train of pulses. By using a frequency multiplier or divider a wave of the frequencydesired for the 4otherrpulseftrain may be obtained. The pulses may thereafterbe produced from'these two waves according to well known methods.

While I have shown and described particular embodiments of the present invention, it isrec ognized, of course, that many variations and different embodiments may be made without departing from my invention. It is therefore to be understood that this description is given by way of example only and not as a limitation on the scope of the invention as set forth in the objects thereof and the accompanying claims.

I claim:

l. A method of multi-channel transmissie comprising producing a train of pulses, segregating spacedsegments from the signal energies of a plurality of channels, differently timing the segments of the different channels to interleave them together into a series of segments wherein each segment corresponds to a pulse of said train.,

' 45 and time modulatingr the pulses of said train acand timemodulating the pulses of said train ac-' cording to the amplitude of the corresponding signal pulses.

3. A method of multi-channel transmission comprising producing a train of pulses of a given repetition frequency, dividing the pulse energy of said train to obtain a series of pulses of a lower repetition rate such as may be desired for chan- 65 nel communication,V retarding by diierent amounts the energy of said series of pulses to obtain a plurality of series of Vpulses of different timing so that they interleave in time similarly `as the pulses of said train, translating .the pulses of each series into signal pulses the amplitudes of which vary according to the instantaneous valuesof the signal energy of the corresponding channel, and time modulating the pulses of said train VVaccording to the amplitude of the corresponding signal pulses.

4.'A method of multi-channel transmission comprising producing a series a pulses of a repetition rate desired for channel communication, multiplying the pulse energy of said series to obtain a train of pulses having a repetition frequency corresponding to a given plurality of channels, retarding by diierent amounts the energy of said series of pulses to obtain a plurality of series of pulses of different timing so that they interleave in time 'similarly as the pulses of said train, translating the pulses of each series into signal pulses the amplitude of which Vary according to the instantaneous values of the signal energy of a corresponding channel, and time modulating the pulses of said train according to the amplitude of the corresponding signal pulses produced from said plurality of series.

5. A method of time modulating pulses comprising producing a rst train of pulses varying in amplitude according to the instantaneous values of a given signal energy, producing a second train of pulses having a repetition frequency corresponding to the repetition frequency of the amplitude modulated pulses, and time modulating the pulses of said second train according to the amplitude values of the corresponding pulses of said rst train.

6. A system for time modulating pulses Cornprising means for producing a train of signal pulses of a given repetition frequency, the pulses of which vary in amplitude according to instantaneous values of a given signal wave, means for producing a second train of pulses having said repetition frequency, and means for varying the time positions of the pulses of said train in accordance with the amplitude values of the corresponding signal pulses.

7. A multi-channel transmission system comprising means for producing the plurality of series of pulses differently timed so that the pulses of said plurality of 'series interleave in time, means for producing a train of pulses corresponding to said plurality of series taken together, means for translating the pulses of each series into signal pulses the amplitudes of which vary according to the instantaneous values of the signal energy of a corresponding channel, and means for time modulating the pulses of said train according to the amplitude o1 the corresponding signal pulses.

8. A system according to claim '7 wherein the means for translating the pulses of each series into signal pulses includes a normally blocked amplier for the signal energy of each channel and means to apply one of said series of pulses to unblock each said amplifier for the duration of each pulse of such series.

9. A system according to claim '7 wherein the means for time modulating the pulses of said train includes delay means for retarding the time 8 position of the pulses and means for applying the signal pulses of varying amplitude to said delay means to vary the retardation effect thereof on the pulses of said train.

10. A system according to claim 7 wherein the means for time modulating the pulses of said train includes means for producing a zig-zag shaped wave from said train thereby providing triangularly shaped undulation having at least one inclined edge to correspond to each pulse of said train, means for gate clipping the undulations between two levels, means for varying the clipping levels of the gate clipping means relative to the amplitude of eacn undulation according to the amplitude of the corresponding signal pulse, and means for differentiating the clipped portions of the undulations to obtain pulses, the time positions of which represent the time position of the clipped portion of .the corresponding inclined edges of said undulations.

11. A multi-channel transmission system comprising means for producing a rst train of pulses, means for producing a second train of pulses the repetition frequency of which is a multiple of the repetition frequency of said iirst train of pulses, retardation means for retarding the pulse energy of said first train 0i pulses to produce a plurality of 'series of pulses diierently timedl so that the pulses taken together correspond to the pulses of said second train, separate means for varying the amplitude of the pulses of each series according to the instantaneous values of the signal wave of a corresponding channel, and means to time modulate the pulses of said second train according to the amplitude values of the pulses of said plurality of series.

12. A system according to claim 11 wherein the means or'producing the pulses of the second train includes means for multiplying the pulses of said rst train.

13. A system according to claim 11 wherein the means for producing the pulses of said first train includes means for dividing the pulses of said second train.

14. A system according to claim 1l wherein the means for varying the amplitude of the pulses of each series includes a normally blocked amplier for the signal energy of each channel and means to apply one of said series of pulses to unblock each said amplier for the duration of each pulse of such series.

15. A system according to claim 1l further including means to produce a synchronizing pulse channel comprising means for mixing the pulses of said first train with the time modulated pulses of said second train, the pulses of said first train being distinguished in width from said time modulated pulses.

EMILE LABIN. 

